Dynamic Controls for Static
Learn about what static electricity is, the factors that affect it, and the methods available to counteract it.
If your prints suffer from ink-adhesion problems caused by surface contamination from dust and particulates, or if you’re having issues with media sticking, binding, or misrouting, you may be shocked to learn that your issues are likely rooted in static electricity.
What is static electricity?
When a material or object holds a net electrical charge, either positive or negative, it is said to have a static charge. The term static is a relative one, as in many cases static charges will slowly decrease over a period of time. The length of time depends on the resistance of the material. For practical purposes the two extremes can be taken as plastics and metal. Plastics generally have very high resistivity. This allows them to maintain static charges for long periods of time; on the other hand, metals have very low resistances and an earthed (grounded) metal object will hold its charge for an imperceptibly short period of time.
Factors that affect static electricity
Many factors affect the generation and maintenance of a static charge. Among them are humidity, the type of material, repetition, and change in temperature.
Type of material Some materials are more readily charged than others. For example, materials such as acetate gain a charge very readily, while glass gains a charge less readily. Also the relative position of materials on the triboelectric series will determine whether a material charges positively or negatively dependent on the other material with which it has come into contact. For example, hard rubber, when rubbed against nylon, will become negatively charged but will become positively charged when rubbed against polyethylene.
Humidity Generally speaking, the dryer the environment, the higher the level of static charge and, conversely, the higher the humidity, the lower the static charge. In relative terms, water is a significantly better conductor of electricity than most plastics. Atmospheric humidity deposits small quantities of water on all surfaces in the environment; therefore, surface static charges on materials have a tendency to dissipate to earth by current flow through the surface moisture.
Repetition Repeated actions, such as friction or separation, increase the level of charge found on a material. For example, a plastic web moving over a series of Teflon rollers will increase its surface charge after every roller.
Battery effect The combination of many charged items can lead to extremely high charges. For instance, individual sheets of plastic with relatively low surface charges when stacked together can generate extremely high voltages.
Change in temperature As a material cools down it has a tendency to generate charge. The action of the cooling is to leave a net charge on the material throughout its entire volume. If the material is a very good insulator the internal (volumetric) static charge can be maintained for extremely long periods of time. However, over time this charge normally migrates to the surface, at which point it becomes a surface static charge.
Methods of elimination
The fundamental principle for neutralization of static charges is the same, regardless of the technique used. Where a material has a positive surface charge electrons must be delivered to the surface to bring the charge back into balance. Where the surface charge is negative the excess electrons must be removed from the surface to neutralize the charge.
Humidity Moisture on (or within) a material will tend to leach away static charges down to earth. For example, paper generally has relatively high moisture content and does not maintain particularly high levels of static. However, if the paper is particularly dry, static can become a severe problem.
Passive ionization The close proximity of a conductor to a charged object will tend to discharge it. For example, a carbon-fiber brush will reduce static charges in materials passed in close proximity to the brush.
Radioactive ionization Radioactive sources cause ionization of the surrounding air, thereby neutralizing surface static charges. A drawback of radioactive eliminators is the radioactive source loses its effectiveness over time and requires replacement on an annual basis.
Active electrical ionization Ionized air can be produced via high-voltage AC or DC, which can then be used to neutralize surface charges. The use of AC or DC systems is application dependent.
Passive eliminators A charged object generates an electric field between itself and any surrounding earthed object (or any object of differing voltage). In the case of a passive eliminator, the field is between the surface and the tips of the carbon-fiber or stainless-steel earthed brush. The fine point at the end of the individual bristles causes the electric field to be highly concentrated at this point. Ionization of the air molecules surrounding the tip occurs when the strength of this electric field reaches a sufficient value.
Radioactive eliminators Radioactive eliminators use polonium-210 or another low-level radioactive source. Alpha particles are emitted to the surrounding atmosphere in the process of radioactive decay. These high-speed particles collide with the air molecules and, in doing so, cause the air to become ionized. This ionized air then neutralizes nearby surfaces in similar fashion to the passive eliminators.
AC eliminators AC Eliminators operate at supply frequency. The main voltage (110, 240, etc.) is greatly increased using a ferro-resonating transformer to generate voltages of between 4.5-7 kV. This high voltage is fed to the ionizing pins, and the casing of the bar is connected to earth.
If we look at the positive cycle of the input waveform, we will see that the electrode pin is at a positive voltage compared to the casing. This generates a strong electric field between the two that is highly concentrated at the sharp point of the electrode pin. This, in similar fashion to the passive bar, generates positive ions at the pin point. These molecules are then repelled from the pin due to their like charge. As the ionization at the bar is not dependent upon the surface charge and ions are produced regardless of the proximity of a surface charge, complete neutralization of a surface can be achieved. This is a significant advantage over the passive eliminators.
Pulsed DC eliminators Pulsed DC eliminators, like their AC counterparts, produce ionized air by using high voltage. Whereas the AC units operate at supply frequency the pulsed DC units operate at lower frequencies, often between 0.5-20 Hz. The low frequency of operation lends pulsed DC equipment to long-range neutralization. The ionizing bar consists of a series of emitters connected alternately to the negative and positive outputs. The casing of the bar is made of plastic, so there is no proximity earth.
The output from the power supply is effectively a square wave switching from negative to positive at the chosen frequency. Looking at the positive half of the wave form, the controller switches on the high output voltage connected to the positive emitters. This then sets up an electric field between the emitter and the surrounding earthed objects. At the sharp point of the emitter this field is extremely strong, and in similar fashion to the AC eliminators, positive ions are produced. The similar charge of the ion and the emitter drives the ions away from the bar. Long-range pulsed DC bars can be very effective in screen-printing production by eliminating electrostatic charges from screens without causing drying.